348 backfires while shifting

My 94 348 has just come out of a full major. Initially there was no backfiring out the exhaust when shifting. After approx. 150 miles I get one or two pops when shifting, I think out of the right side exhaust but not certain on this. Otherwise the cars starts good, runs great, and has no lights on. Would appreciate any ideas on what to check.

Thanks,

Brewman

 

Forgot to mention both MAFS. Set at 384 ohms. Thanks and any ideas on further checks appreciated.

Brewman

 

And your CEL lights are actually working? -- i.e., they pass the self-test at first key "on", or are they just always (wrongly) "off"?

An air leak into the exhaust system (either directly or via the air injection system, if so equipped) can cause such a symptom (yet still have good run-ability).

 

Steve,

The CEL lights do come as they should, pass the self test and then go out. Will check the exhaust system. Thanks for the input.

Joe

 

Isn't this a classic symptom when the "non-return valve" (Tav 8, item 4) is ruptured or stuck open? It's supposed to allow fresh air to be pulled into the exhaust manifold(s), but if it isn't working properly the exhaust pulses run back into the intake system. Seems to me that there is a generic replacement for the ridiculously spendy original Ferrari part.

 

Yes, or item 8 (the cutoff valve) on that same page, but I thought Motronic did have some intelligence in that regard (and would trigger a CEL for such a fault -- but don't know that for sure).

 

That typically allows reversion of the exhaust gasses and melts the hoses and shutoff valve.
In my experience it is more often a malfunction of the air shutoff valve (#8), usually misconnected vacuum hoses. That also causes the O2 sensors to go full rich and melt the cats, if there are still there. I have seen a lot of cars with a history of cat problems that were caused by improperly connected vacuum hoses for that shutoff valve.

 

Sadly no.

If memory serves OBD2 was the first and only looked to see if air pump was running. Did not look to see if it was not running.

If it is indeed a shutoff valve continuing to supply air it is not an uncommon problem on the 348/Mondial t.